Mysterious Vanishing Disk Space — Solved

If you ever find that your Mac is mysteriously losing gobs of internal disk space, try this:

1. We are about to do some serious deleting, so take precautions. Unmount and eject any external hard drives, CDs, DVDs, network drive shares, etc. Physically remove the discs/disks or disconnect them from your Mac so you can be sure that you won’t inadvertently lose any files. To be extra safe, consider turning off WiFi momentarily or disconnecting your Ethernet connection.
2. In the Finder, choose Go to Folder… from the Go menu (or press command-shift-g).
3. In the dialog box, type /Volumes and press return or click OK.
4. You should be looking at a Finder listing of all mounted volumes on your computer. If you see a folder with the name of a disk that is not mounted, you can be pretty sure that OS X has surreptitiously copied all or part of an external drive to your internal drive.
5. Move the suspect folder to the trash.
6. Empty the trash.

I just recovered about 30 gigabytes of space this way.